Optical networks carry information in the form of light over optical fibers. Various forms of wavelength division multiplexing (WDM) can be used to carry multiple optical signal channels over a single optical fiber, specifically by conveying each optical signal channel using a different optical wavelength. Optical signals can be switched using various types of networking devices, such as optical add/drop multiplexers, optical cross-connect switches, and routers.
Nodes in optical networks can be generally categorized as being either opaque or photonic. An opaque node is one in which optical signals are converted into electronic information for processing in the electrical domain. A photonic node is one in which optical signals are processed in the optical domain.
Photonic switching nodes can perform different types of switching functions on optical signal channels. One switching function, referred to hereinafter as “photonic pass-through,” switches an optical signal channel from an input fiber to one of a number of output fibers. Another switching function, referred to hereinafter as “add/drop multiplexing,” removes (drops) an optical signal channel from an input fiber and/or adds an optical signal channel to an output fiber. The dropped optical signal channels can be directed toward one or more clients. The added optical signal channels can be provided by one or more clients.
Photonic pass-through and add/drop multiplexing can be accomplished using a large photonic cross-connect switch (PXC) that is connected to the input and output fibers as well as to various add/drop interfaces. Such a large PXC is typically very expensive.
Photonic pass-through and add/drop multiplexing can also be accomplished using a number of small switches, each dedicated to a single wavelength or waveband, and each having a number of line interfaces and a number of add/drop interfaces that operate on its respective wavelength or waveband. A photonic switch that uses this approach is referred to hereinafter as a “wavelength plane switch” or “WPS.” A special case of this design uses 2×2 switches or a pair of 1×2 switches to connect two lines to two add/drop interfaces in an optical add/drop multiplexer (OADM) configuration.